Lithium niobate crystals are known as electro-optic materials that are useful as holographic recording media which have fairly good sensitivity. This sensitivity, as well as diffraction efficiency, can be greatly improved by doping such crystals with iron, as has been disclosed in U.S. Pat. No. 3,703,328 to Glass et al.
In such applications, it is generally advantageous to maintain a minimum function of iron ions (Fe) in the divalent state (Fe.sup.+2), in order to achieve an optimal holographic write sensitivity, and a relatively larger function of Fe ions in the trivalent state (Fe.sup.+3), in order to minimize photoconductivity and, hence, minimize sensitivity to self-erasure effects when large numbers of holograms are time-sequentially stored within a common volume of lithium niobate. By maximizing write sensitivity and minimizing erase sensitivity, more high-efficiency holograms can be stored and greater information densities achieved within a given volume of iron-doped lithium niobate (Fe:LiNbO.sub.3).
Problematic with such iron-doped lithium niobate crystals, however, is the difficulty in producing such crystals wherein the iron portions contained therein exist predominantly in the trivalent state. More specifically, significant difficulty arises in oxidizing the divalent iron ions normally contained within such crystals, insofar as conventional oxidizing agents and techniques fail to thoroughly penetrate such crystals and hence oxidize the iron contained therein.
While certain processes are known in the art that are effective in oxidizing substantially all of the divalent iron ions present to trivalent ions, such processes suffer from other drawbacks. Generally, most prior art processes require heating such crystals to extremely high temperatures for prolonged periods of time. Such heating, which is typically carried out at temperatures between 800.degree. C. and 1100.degree. C., can cause the formation of lithium tri-niobate (LiNi.sub.3 O.sub.8), which, as is known to those skilled in the art, causes light to scatter and thus renders the crystal useless in holographic applications.
Such prolonged heating further is known to cause a portion of the lithium present to diffuse out of the crystal. Such diffusion, which is known to begin to occur at approximately 900.degree. C., causes light-absorbing color centers to form within the crystal. The light-absorbing properties of these centers dramatically limit the ability of such crystals to function properly in holographic applications, especially insofar as such color centers are known in the art to absorb light having a wavelength around 532 nanometers, which, for system reasons, is near-optimal with regard to recording holographic data.
Accordingly, there is a need in the art for an efficient, more thorough process for oxidizing iron-doped lithium niobate wherein the trivalent to divalent iron ion ratio is greatly enhanced. There is also a need in the art for a process for oxidizing iron-doped lithium niobate such that the iron ions present may be substantially oxidized from the divalent state to trivalent state that avoids the formation of unacceptable levels of lithium tri-niobate and further substantially minimizes the diffusion of lithium from the lithium niobate.